Kris is right; for a '73 you best contact Mica and maybe post a pic of the screws that you do have. For '76 forward, these are the screws to get; but as indicated on that page, prior to '76 some instruments had wood screws for the back plates rather than stainless steel into threaded inserts.

I'm not sure if this will help you, but my 78 series one has the older style wood screws and I had a similar problem. Part of the reason why you may be missing three is because these often seem to strip out from the wood over the years.

I got around this problem by using Tamiya epoxy modelling putty (from a plastic modelling shop). This is easy to fill the old screw holes with, and you can poke a small hole in it when it is still wet with the sharp end of a toothpick. worked very well on my series.

After this putty had cured (with the pre-formed holes), the screws went in very well and held tight.

The first thing I noticed is the pickup height adjustment screws could be reconfigured . From the factory Alembics have two screws above the ears of the pickups and two below. The screws below the ears determine the desired height and the screws above hold it snug, you want to be very careful not to tighten the upper screws too tight as that can crack the pickup enclosure.

Here is a pic of what I mean:

The screws on the treble side of the pickup are opposite of the bass side , this picture illustrates that well:

If you want to take a close-up of the screw inserts under the back plate that would help to make sure that you get the right replacements, If the picture is over 150 kb then it will need to be compressed. I'd be happy to do that if need be.